Transmission system



March 1946- R. H. HERRICK 2,396,175

' TRANSMISSION SYSTEM Filed May 1, 1943 4 sheets-sneak 2 I INVENTOR. ROSWELL H-HERRICK ATTQRN EY S March. 5, 1946 I R. H. HERRICK j I 2,396,175

TRANSMISSION SYSTEM Filed ma 1, 1945 4 sheet -sheet .3

v 340 33 A c 'ZZOL .fl, I I all,

, I .INVENTOR. v ROSWELL H. HERRICK ATTORNEYS Patented Mar." 5, 1946 TRANSMISSION SYSTEM Roswell n. Herrick, Oak Park, 111., assignor to Automatic Electric Laboratories,'lnc., a corporation of Delaware Application May 1, 1943, Serial No. 485,268

17 Claims.

The present invention relates generally to improvements in signal current transmission systems of the type in which signal controlled switching circuits are provided for partially or fully block n under certain conditions, certain of the signal channel included therein and to increase the gain of certain ,of the other signal current channels, and, more particularly, to improvements in telephone substation circuits having coupled signal current channels for the transmission of incoming and outgoing signal currents.

Heretofore; telephone substation circuits have been provided with a hybrid system or anti-sidetone impedance networks for preventing signal currents developed during operation of the trans-- mitter from being transmitted to the receiver and for similarly preventing signal currents incoming over the line extending to the substation from being transmitted to the transmitter for reproduction. In such arrangement one of the factors which determines the efllciency of the sidetone suppression is the impedance of the talking circuit established by wayof two connected subscriber lines. Since the impedance is not the same for any two different established connections, the hybrid system of each substation is usually balanced to provide maximum sidetone suppressi n emciency for average line conditions.

In installations .wherein amplification of incom ing and outgoing signal currents is required, as, for example, in executive loudspeaking sets, the

conditions of unbalance introduced in the substation circuit by the impedances of the lines over which an established connection extends may become intolerable. true in substation installations provided in an exchange area wherethe subscriber lines are of or the usual telephone type of communication could be obtained selectively. 7 Such an arrange! ment included means whereby rapid change-over could be made between the two types of conversation without appreciable interruption. In order that speech clipping might be minimized the amplifying channels were provided with control circuits arranged to be exceedingly fast in operation. In order to provide such fast operation it was found necessary to reduce the time delay factor encountered by the enabling circuit equipment which, however, ma'de the equipment more susceptible to feedback between the control circuits and between the channel amplifiers. The

advantage of a relatively rapid acting control circult which would minimize speech clipping and which also would retain the advantages of v preventing feedback between the diflerent components of the apparatus has been made possible by certain circuit arrangements and by the use of various types'of tubes.

The present invention is directed to channel amplifiers and control circuits for telephone substation apparatus which provides selective communicationby the usual subset or by an executive loudspeaking telephone apparatus. In providing This is particularly conventional telephone circuit or as an executive loudspeaking telephone circuit.

use, in other arrangements the signal channel not being used is rendered comparatively inactive. Apparatus of the type in which the channels are only partially blocked when not in use have .e advantage of permittingv conversation operation always included the use of such amplifiers. Since this did not always provide the desuch arrangements, it furthermore would be desirable to minimize the number of types of tubes used in such equipment, thereby'to simplify servicing and replacement of tubes.

It is an object of the present invention to provide an improved telephone substation apparatus which is adapted to be operated either as a It is another object of this invention to provide an improved telephone substation circuit transmission system having a plurality of channel ampliflers provided with control circuits sumciently fast in operation'to minimize speech clipping, and

yet adequate provisions for the prevention offeedback are provided.

Still another object of this invention is to provide a pair of channel amplifiers and associated control circuits which utilize a minimum number of different types of tubes.

A still further object of this invention is to provide a channel amplifier having a plurality of stages of amplification and a control tube therefor, whereby. the operation of the channel amplifier iscontrolled by a combination of conor acting to produce accumulative effects.

In the illustratedembcd-iment of the invention there is provided a telephone'substation 'apparatus of the type adapted to be connected to a associated telephone line. This apparatus includes the'usual transmitting and receiving telephone subset, a transmission system comprising a hybrid-system or network, a microphone, a loudspeaker, and a pair of channel amplifiers together with control tubes therefor.

for operation to rendereffective the loudspeaking equipment. Means are also provided for subsequently rendering ineffective the loudspeaking communication channels when it is desired to communicate by the conventional telephone subset.

' Each of the channel amplifiersis provided With a control circuit responsive to signal currents; passing through the channel amplifier for increasing the gain of that channel and for decreasing the' gain of the other' channel. Automatic volume or gain control means are provided which have a timeconstant sufficient to maintain the gain of the amplifier so as to hold over word and syllable pauses. To provide an exceedingly rapid enabling action by the control circuits, an initial control voltage is applied to the associated amplifier which is obtained from means having a minimum filter action, and subsequently adequately filtered voltage is supplied to the amplifier to prevent feedback between the amplifier channels. In order that a minimum of time be required to condition the amplifiers for use. the amplifiers and control tubes are equipped with tubes requiring low thermal filament energy. and each control tube and each group of amplifier tubes are supplied from individual sources of direct current to prevent feedback between the various circuits through the filament sources. r

In accordance with the present invention each of the control ci cuits. in response to signal currents passing through the associated amplifier,

. generates a control voltage by means having a -relatively short time constant,.thereby to rapidly increase the gain of the associated channel amplifier. Subsequently a relay operates to render inefiectivethis i itial control volt ge and to apply a smaller negative blas'to the channel amplifier.

Other and further features of this invention will sub eouent-ly become apparent in the following description. v

The novel features believed to be characteristic of the invention are set forth with particularity in the ap ended claims. The invention, both as to its org nization and m thod of operation, together w th further objects and advantages thereof, will best be understood by reference to thefollowing description taken in connection with the acc mpany ng drawings, in

which Fi s. 1 to 4. inclusive, illustrate t e substation c cuit having incorporated therein ce tain of the features-of the invention as briefly outlined above.

tra t-es the app ratus provided at a telephone sub- More specifically. Fig. 1 illus-' telephone subset.

' trol voltages acting in opposition to each other trates the powerequipment for supplying the a a necessary operating and controlling potentials for the amplifie apparatusj and Fig. 4 illustrates the hybrid system and control relays for selectively controlling the interconnection of the substation amplifier apparatus and the. substation telephone line.

Referring more particularly to Fig. l of the drawings, there is shown a telephone subset. comprising a transmitter. IN; a receiver .I02, :1 receiver switch I03, induction coils I08. I09 and I I I, and an isolating or'coupling capacitor H2. The subset'maybe provided with a dial switch H3 and also witha ringer I22 and a ringing ca pacitor I24. Thesev parts constitute the conventional elements .commonlyfound in a subscriber The standard subset is arranged for selective use with an executive type for operation the loudspeaking equipment.

loudspeaking set. The' selective operation of either type of communicationjwhich "will subsequently become apparent, is brought about in part by a switch I25 which operates to condition The loudspeaking equipment includes a microphone I33 and a loudspeaking telephone I34. I

The telephone transmitter IOI, the receiver I02, the induction coils I08, I09'and III the calling device H3, and the receiver switch I 03' are interconnected to form a substation. circuit of the well known anti-sidetone type; This circuit includes the line conductors H0 and I20 between which the three inductively coupled windings I08, I09 and III are seriallyincluded in a branch circuit. This branch circuit also includes an isolating capacitor -I I2- connected between he two windings I08 and II I, the-capacitor being effective to prevent the receiver from being energizedby the direct current flowing over the ings I09 and III, it. is protected against high voltages appearing upon the lin: comprising the conductors II 0 and I20, which voltage surges might result in puncturing the insulation of the capacitor.

For the purpose of giving an audible indication of calls incoming to the substation, there is ,vice II3.

scriber substation and incl des loudspeaking apparat s and switch ng equipment for selective operation of the substation as. a regu ar teleprovided a. signal devicedn the form of a ringer I22 which is norma'ly connected to be energizedover an operating circuit including the line conductor IIO, the capacitor I24, the contacts H4 and H5 of the calling device 113, and the contacts I3I. and I32 of the switch I25 which are co nected to the other line conductor I20. The induction coil I08 which-has one terminal connected. to the juncture. between the transmitter I 0! and the capacitor II2, has its other terminal connected to the make. contact I28 of the switch I25 and also to a resistor I23 which is connected to the contact I I6'of th calling dethe switch I25 and the resistor I23 will subsequently become appare t.

phone subsc iber substation or as a l u'dspeaking type ubstation: Fig, 2 illustr tes the outgoing amplifier. the incomin amplifier and the control appuatus at the substation; rig. '3 illus-r The calli g device I I3 is provided with a make contact IIB which cooperates with the contact II5 which, it will be remembered, was connected to the capacitor I24. The calling device, further-- mor has a. pair of makecontacts III and H8 The portion of the branch cir- The purposes of the contact I28 on description.

- become apparent.

assume which are connected between the line conductor I20 and the common juncture between the resistor I23, the inductance coll I98 and the switch contact I28 of the switch-I25. Upon actuation of the calling device H3 a pair of interrupter contacts H9 and I2I- are arranged to be broken to provide the calling impulses. These contacts are connected between the switch contact H8, which is connected to the common juncture of the resistor I23, the inductance coil I08, the

switch contact 828, and the other line conductor 7 Hit. When the dial or calling device 3 is actuated the ringer I22 is disconnected from the conductors H ii and Iilu which comprise the telephone line, and the capacitor IE5 is connected through the resistor its so as to be in parallel to the interrupter contacts H9 and EM, thereby to prevent or minimize transient efiects occurring as a result of the making and breaking of the contacts I01 have also been closed so as to prepare 9. eh I cult which is arranged to operate to render ineffective, the hybrid system connected to the tele- I phone line I I0 and I20 ii the loudspeaker set is l in operation. This circuit operation also will subsequently-become apparent in the following The contacts I05 and I01 of the hook switch I03 are connected to the make contacts I28 and I21 of the switch I25, the latter:

The calling device or dial mechanism H3 is also arranged to become eflective upon actuation of the switch I25 which will close the make contacts I28 and H29. the latter contact being connected to the conductor I30 which is connected to a hybrid system interposed between the line connected to the conductors I90 and 208 which are connected across the input coupling resistor 209 connected to the grid of a vacuum tube 20!. The microphone I 33 is therefore coupled to an outgoing channel amplifier which is provided with a control tube, and which includes triode vacuum tubes 20I, 202, 203, 2% and 20 5. The anode of the triode 2i" is connected to the primary winding of a transformer 2i I. The secondary winding of the transformer 2H is shunted by a coupling resistor 2I2 and one terminal thereof is connected to the grid of the vacuum tube 282. Similarly, vacuum tube 262 is coupled to vacuum tube 203 by means of a transiormer 2I8 and coupling resistor 2M; and, likewise, tube 203 is coupled through the transformer 2th and con pling resistor 2I6 to the output vacuum tube. 204. One terminal of the primary winding of each of the transformers M I, 253 and are is connected to the conductor Iiuwhich supplies a suitable anode potential. The anode of the vacuum tube 20 i is connected to the conductor are which, in turn, is connected to a hybrid system it! shown in Fig. 4, which is also connected-to the conductor 220. The conductor 220 is also connected tothe relay I43, and hence between the conductors 2H3 and 220 there appears a choke coil 2 I1, a voltage divider resistor 2 I8, and a second voltage divider resistor 2I9. An adjustable contact of the voltage dividerresistor 2| 9 is connected through a capacitor 22I, a current limiting resistor 222, and a dry disk or contact rectifier 223, to a resistor 225 which is connected to ground and which is by-passed by'a capacitor 224. The juncture of the dry disk rectifier 223- with the capacitor 224 and the resistor 225 is connected to resistors 226 and 228, which together with capacitors 221 and 229 constitute a time delay circuit connected to the grid circuit of the input vacuum tube 20I. The capacitors 221 and 229 are connected between ground and opposite terminals of the resistor 228. The movable contact onthe resistor 2I9'.

. takes a certain amount of the output voltage of conductors III) and I20, and the two channels connected to the microphone I33 andthe loud-- speaker I34. Upon actuation of the switch I25 the break contacts I3I and I32 are opened so as to disconnect the ringer I22 from across the line conductors M0 and I20. I

The microphone I33 is connected to a pair of conductors I52 and I60 which, in turn, are connected to the primary winding of a transformer I25. Across the conduct rs I50 and I60 there are connected resistors I35 and I36, the mid oint of which is connected through a signal device or lamp .I 31 to ground. At the other end of the conductors I52 and I 9. pair of resistors I38 and 139 are connected, the midpoint thereof being connected to t e make contacts IQ'I and I42 of a relay its. conductor I00 which, in turn, is connected to a suitable source of voltage. The relay l43. which the vacuum tube 204. which is passed through the dry disk rectifier 223, thereby to develop across the resistor 225 and the capacitor 224 a certain amount of direct current voltage, which is a 7 function of the signal current being transmitted The contact 5 32 is connected to a is shunted by a capacitor M4, is connected between the conductors 220 and I110. The conductor22t is connected to the output vcircuit of the outgoing channel amplifier and the hybrid and which is used to supply an automatic gain control voltage to the input tube 20L The manner in which this arrangement functions will more readily be understood after the remaining circuit elements have been described and a fuller explanation will be given in connection with a description orthe operation oi the system. The

grid return of the vacuum tube 202 includesa resistor 23l, by-passed to ground by a capacitor 202, and a conductor 230 which is connected to a circuit subsequently described. The grid re-v turn circuit of the vacuum tube are includes a resistor 233 connected to ground, and a connection to contact 2M of a relay 286. The secondary winding of the transformer 2 I5 and the coupling re'sistor m of the vacuum tube 2% are connected directly to ground.

A portion of the voltage appearing-acrossthe resistor H8 is coupled through the capacitor 238 to the grid of a vacuum tube 2% which is provided with a grid coupling resistor 23%? having one end thereofconnected to ground. The anode of the vacuum tube 225 is connected to a capacitor 236' which is grounded and to a relay 231 which is connected to the conductor I10 which supplies anode voltage. The relay 237 is pro- .vided with make contacts 24I and 242 connected .a a assure between the conductor lit and a slow release relay 288 which. in turn, is connected to a. conductor 266 which'is connected to the low potential side of a source of voltage. The relay 243 is provided with make contacts 263 and 264 connected to conductors 266 and 210 which are connected to a circuit subsequently to be described. The relay 248'is provided with a capacitor 244 connected between the conductor 200 and through the break contacts 246 and 246 and a current limiting resistor ills-to the conductor I which is connected to the positive side of the high voltage source. The contact 266 is arranged to makewith-contact 261 so as to connect the capacitor 246 across the relay 2243, thereby to provide and enhance a slow release characteristic. The slow release relay 243 is also provided with make contacts 248and 248 which are arranged .to disable the incoming channel amplifier. V

The incoming channel amplifier is energized from a voltage divider 266 connected across the incoming leads 290 and 300 which are connected to the hybrid system I of Fig. 4. The input of the vacuum tube 206 is connected to the voltage The adjustable contact on the voltage divider 268 is connected through a coupling capacitor 2", a current limiting resistor 212, and a dry disk rectifier 216 to a'resistance capacitancecircult comprising a capacitor 214 connected across a resistor 216. A portion of the signal voltage appearing across the voltage divider 268,- therefore, is rectifiedby the dry disk rectifier 213 to produce across'the resistance capacitance circuit 214; 216 a direct current voltage proportional to the signal current passing through the amplifier.

240 which is connected to the negative side cia high voltage source of power. A capacitor 29!, connected through the break contacts 286 and 286 of the relay 266 in series with a current limit- ,ing resistor 286, is connected between conductors Thisvoltage is applied to one terminal of the grid coupling voltage divider-286 of the vacuum tube 206 through a resistance capacitance filter circuit comprising the resistors 216 and 218 and ed to a contact 265 of a slow release relay 286 and also to a conductor 330 which extends t the source of voltage shown in Fig. 3.

A portion of the output voltage app aring across the voltage divider 269 is connected through a coupling capacitor 291 to the grid of the control tube 268- which is provided with a grid coupling resistor 286 connected between the grid and ground. The anode of the control tube 266 is connected to a grounded capacitor 282 and to a. relay 283 which is energized from the vhigh potential conductor E10. This conductor is also connected to the contact'284, of the relay 283. which is arranged to make contact with contact 286. Contact 266 is connected to relay 286 which. in turn, is connected to the conductor tor 29B across the relay 266, thereby to provide and enhance the slow release characteristics of this relay. The slow release relay 286 is also provided with make contacts 286, 294 and 283, the

latter contact being connected to an adjustable resistor 262 which, in turn, is connected to the conductor 240. The contact 284 is connected to a conductor 840 which extends to the power supply circuit shown in Fig. 3. The slow release relay 286 base contact 26 I connected to the input circuit of vacuum tube 203 which is arranged to break with the contact' 269 connected to ground, and to make with contact 262 which is connected to the grounded capacitor 268, and a resistor 266 which, in turn, is connected to an adjustable contact on the voltage divider 266 and to grounded capacitor 261. A voltage diyider 264 has an adjustable contact connected through a resistor 262 to the make contact 249 of slow release relayv 2'43. Suitable filter capacitors 26I and 263 are connected between ground and opposite terminals 01 the resistor 262. The

voltage dividers 264 and 266 are each connected between ground and a conductor 260 which is connected to the negative side of a low voltage source or direct current.

Reference may now be had to Figs. 3 and 4, which show the incoming line circuit and the source of power which supplies anode. grid and filament voltages for the various tubes. The telephone line I20 is connected to the hybrid system 40I which preferably'is composed of a pair oi! transformers 402 and 403. eachhaving a pair of secondary windings 404, 406 and 406, 401, respectively. The conductor I20 is therefore connected through the secondary winding 406, a relay 408 shunted by a capacitor 408, and secondarywindmg 404 to the make contacts M6 and 4" of a relay 4I3, the contact 4| 6 being connected to a conductor I30. It will be remembered that the conductor I30 extendsto Fig. 1 where the conductor is connected to the make contact i2 9 of the switch I26. The remaining secondary'windings 406 and 401 of the transformers 402 and 403 are reversely connected through a balancing impedance or resistor 421. The primary winding of the. transformer 403 is connected across the conductors 290 and 300, so as to energize the voltage divider 266 which is connected to the gridof the input tube 206 of the incoming channel amplifi r. The conductors 290 and ll'are also connected to the make'contacts M8 and 388 of .the relay 603. The primary winding of the transformer 402 is-connected to the conductors vFig. l is actuated to close the contacts lid and I29, the conductor I30 isconnected to the line conductor H0 through the contacts H9 and i2! ofv the dial device H3. When this circuit is completed the relay 408 and the transfgrmer.

windings see and 406 are connected across the line conductors H0 and I20, whereby current fiows to energize the relay 408. The operation of ne ates the relay see closes the contacts an and dig, thereby to energize the relay ifii from a suitable alternating current source, whereupon contacts are and 323 of this relay are closed, thereby to supply alternating current energy across theconductors 3i0and 320 which, in turn, are connected to the primary winding of three power transformers 308, 302 and 303. The secondary winding of the transformer 3i is connected to a bridge rectifier 30d, the'output of which is connected between ground and a conductor 250. In terposed between the low potential side of the terminals. G and H supplies power only to the rectifier network 300 and the conductor 250 is'a filter choke 305, the opposite terminals of which are connected through capacitors 306 and 301 to ground. The rectifier and filter arrangement therefore supply a relatively low voltage direct current to the voltage dividers 25d and 255 of Fig. 2, whereby a certain amount ofnegative biasis provided for application to certain circuits of the channel amplifiers. v

The transformer 302 is provided with a pair of secondary windings 308 and 309 which energize the filament and anodes of a full wave rectifier filter reactor 3I2 and to the negative conductor 240.

The filament power transformer 303 is provided with a plurality of mid-tap secondary windings 3I5, 3I6, 3H, 3I8,'3I9 and HI. The secondary winding 3I5 has output terminals A and B which supply only thefilament of control tube 205. The midpoint of the secondary winding 3I5 is connected through a resistor 322 to the juncture of the conductors 230 and 260 which, in turn, is connected through a capacitor 323 to ground. The midpoint of the secondary winding 3I5 is also connected to a filter bleeder circuit comprising the adjustable resistor 324 and the fixed resistor 325. One terminal of the fixed resistor 325 is connected to ground and the other terminal is connected to the negative side of the source of power provided by the rectifier 3. At opposite terminals of the resistor 324 there are provided grounded capacitors 326 and321.

The output terminals of the secondary winding SI 6, which have been indicated as C and D, supply power only to the second stage vacuum tube 202 of the outgoing channel amplifier. The midpoint of this secondary winding 3H6 is connected through a resistor 32% to the conductor 210 and to an adjustable resistor 33! which, in turn, is connected to the conductors 240 and 280 and the negative side of th anode voltage. A capacitor 320 is connected between the midpoint of the transformer 380 and ground toprovide a certain amount of filter action.

The midpoint of the secondary winding 3H is connected to the resistor 332 which is grounded and which is by-passed by the capacitor'333. The output terminals E and F of this transformer supply filament energy to the vacuum tubes. I, 203 and 204 of the outgoing channel amplifier.

The secondary winding 3i8 having output circuit through conductor I00 which is connected output tube 20'! of the incoming channel amplifier and the midpoint of the transformer is connected through the resistor 23% to ground WhiCh, in turn, is by-passed by a capacitor 335.

The secondary winding aid has output terminals I and J which energize the filament of the control vacuum tube 208. The midpoint of the transformer- 3i9 is connected through the resistor 33% to the conductor 330 which, it will The secondary winding 325 which has output terminals K and L supplies filament current to the input vacuum tube 206 of the incoming chan- The midpoint of the secondary winding MI is connected through a resistor 302 to the conductor 340 which is connected to the make contact 294 of the slow release relay 200.

A by-pass capacitor 34! is connected between ground and the midpoint of the secondary winding 32I. The terminal of the resistor. 362 which is connected to the conductor 340, is also connected through an adjustable resistor 343 to the negative side of the-plate voltage source.

The purpose and function of certain circuit elements and arrangements will now become apparent in the following explanation of the operation of the system. If it is assumed that the handset comprising the transmitter IOI and the receiver I02 is resting on its cradle, so that the receiver switch l03 is actuated to open circuit position, and it is further assumed that it is desired to communicate by means of the executive loudspeaking telephone system, the switch I25 will be actuated. Actuation of the switch I25 breaks the contacts Iill and I32, thereby'removingthe ringer I22 from across the telephone line H0 and I20. At the same time contacts H28 and I29 are closed, thereby connecting the conductor I30 through contacts a and I 2| of the calling device H3 to the line conductor H0 so that current flows through the-transformer coils 404 and 406 of th hybrid system -40I of Fig. 4, thereby to energize the relay 408. The relay 408, having been energized, closes its contacts 4H and 2, thus causing the relay 42I to close its contacts 423 and 422. therefore is connected to the conductors 3m and 320, whereby the power transformers 30L 302 and 303 are energized. The various filament transformer windings of the transformer 30% thereby heat the various vacuum tubes 209 to 203, inclusive, to condition the executive loudspeaking apparatus for use. The anode of the output vacuum tube 204 of the outgoing channel is connected through the conductor 2! 0, the hybrid transformer 402, the conductor 220, the relay I03, and the conductor M0 to the high potential side of the plate supply which is furnished by the rectifier tube 3H of Fig. 3. As soon as the tubes have reached operating temperature, anode current will flow thereby to energize the relayI43 which closes its contacts i4! and M2. lhe closing of the contacts 548 and M2 completes a to the negative terminal of the low voltage direct The alternating current sourceceiver switch 503.

current source supplied by the transformer 30I and the bridge rectifier 304, so that current flows over a simplex circuit superimposed on th microphone conductors I50 and I60. Current therefore enters at the common juncture of resistors I38 and I39 flowing over the microphone conductors I50 and IE and leaving at the common juncture of the resistor I35 and I33 to energize the signal lamp or device I31. I lamp Edi is lighted, this indicates that the channei amplifiers and control tubesare now in condition for use.

The actuation oi the switch me by the closing of the contacts 528 and H29 has placed the calling device its in circuit so that a subscriber may be called by actuation of this device, At the same time the actuation of switch A25 has closed contacts ltd and 82? to prepare a circuit which includes conductor its and contact it? oi the re- The conductor M0 is connected in Fig. i to relay sis which, in turn, is connected to conductor Me which is supplied by power from the bridge rectifier direct current sourcettib. If after communication has been started by the executive loudspeaking system it is desired for purposes'of privacy to utilize the handset, the receiver switch I03 becomes actuated thus closing the contacts I06 and I01, whereby the relay 4 I3 is energized to make contacts 414 and 4l5 to lock in circuit the relay 42I, to break contacts M6 and 4Il to disconnect the hybrid system 408 from the telephone line, and also to short-circuit the input to the incoming signal channel by closing the contacts M8 and 419.

Each of the channel amplifiers is so arranged by supplying 'to certain of the tubes thereof a I relatively high negative voltage, that the amplification thereof isat a predetermined low level which would be insufficient to supply signal currents to the telephone line of the desired magnitude. Each of the signal channels, therefore, is provided with a control tuberesponsive to signal currents passing therethrough, whereby the gain of'one channel is rapidly increased thereby enabling that channel, and the gainer the other channel is decreased thereby disabling the other channel. A portion of the initial voice currents fed to the outgoing channel amplifier appears in the output circuit of the output tube 204 across the voltage divider 2 I8. A certain amount of this When the signal rent of the vacuum tube 205, however, the nosetive bias'supplied by the resistors 829 and 33a is opposed by the bias appearing across the resistor 324. The potential supplied by the resistor 32% is obtained from a resistance capacitance arrangement which has a relatively short time concondition of the channel amplifier to the enabled thereby bringing up thegain of the outgoingchannel amplifier to enabled condition.

The bias of the control vacuum tube 205 is equal to the voltage drops across the resistors 324 and 32-5, the latter resistor carrying plate currents, of the vacuum tubes 2!, 203 and 203.

"Therefore, the net bias applied to the vacuum tube 205 does not change greatly with a change in the anode current of the vacuum tube. 205, even though the voltage appearing across the resistor 324 is directly proportional tothe increased plate current of the control tube. There-' fore, the stabilizing action of the voltage drop appearing across the resistor 325 tends to C0m-= pensate for the application of voice voltage to the vacuum tube 205 and thus to reduce'the rate of increase in plate current in this'vacuum tube; If such an arrangement were not provided there would be a much greater rate of increase in the plate current for the vacuum tube 205 which would be highly undesirable from the standpoint of control tube sensitivity.

Since the potential appearing across the resistor 324 varies in proportion to the signal currents passing through the amplifier, and this bias circuit has a relatively short time constant, it is desirable to eliminate this variable effect by connecting the juncture between the resistors 329 and33l to the juncture between the re'sistors23l and 322, which is accomplished by the make con- 1 tacts 253 and 264 or the slow release relay 243.

signal voltage is impressed across coupling capa'cthe voltage drop appearing across the resistor 324. l The resistor 324 is effectively connected in circuit with the grid to cathode circuit of the vacuum tube 202. A circuit from the grid of the vacuum tube 202 maybe traced through the secondary winding of the transformer 2 which is shunted by the resistor 2I2, through the resistor 23I,,th e resistor 322, the resistor 324, resistor 33I and resistor 329 back to the mid-tap of filament trans-' former winding 3"; which energizes the filament of the vacuum tube 202.- The effective negative bias applied to the vacuum tube 202 therefore is the drop appearing across resistors 329 and 33I when no current is flowing in the vacuum tube 205, With the increase in anode to cathode our- The anode current of the control tube .205 having been increased in response to signal currents passing through the outgoing channel amplifier, brings about energlzatlon of the relay 231 which, in turn, energizes, by the closing of 21115 contacts 241 and 242, the slow release relay 243. Operation of the slow release relay 243 brings about the colsing 'of the contacts 253 and 264, thereby to apply a smaller negative biasto the control of the amplifier tube 202.

At the same time the capacitor 244 is connected in shunt to the slow release relay 243 by the closing of the contacts 245 and 241. This increases the slow release characteristics of the relay. The capacitor 244 is relatively large and therefore would tend to lower the voltage across the relay coil momentarily, were it applied directly without previouslycharging the capacitor, The capacitor 244, therefore, is initially connected by the break contacts 245 and 245 through the current limiting resistor 239, so that the capacitor may be connected in shunt to the relay without lowering the voltage thereacross. The current limiting resistor 239 prevents the flow of heavy charging currents whenever contacts 245 and 246 are made.

which otherwise might disturb the amplifier sufficiently so as to cause a noise or decided change in the level of operation. The slow release relay to the grid of the vacuum-tube L aeearve coming channel amplifier to effectively disable the amplifier. The outgoing channel amplifier in addition to being controlled by the self-biasing arrangements,

the control voltage applied by the control tube 2 03 and the change in negative bias brought about by operation of relay203, is also controlled by the automatic gain control arrangement. This includes the resistor 219 which is connected across the output circuit of the vacuum tube 200. A portion of the potential appearing thereacross is coupled by the capacitor 22! through the current limiting resistor 222 to the drydisk rectifier 223 which builds up a potential across the capacitor 224 which is proportional to the signal current passing through the amplifier. The capacitor 220 is discharged at a relatively constant rate by the resistor 225. This automatic gain control voltage appearing across the capacitor 224 is applied through a time delay circuit comprising the reistors 226 and 228 and the capacitors 221 and 229 The grid to cathode circuit ofthis vacuum tube 20! also includes the self-biasing resistor 332 which is shunted .by the capacitor 333 and which is connected between the mid-point of the transformer winding 3!? and-ground. Therefore, the automatic gain control voltage applied to the input tube 20! is superimposed on the self-bias appearing across the bias resistor 332. The resistorcapacitor-filter arrangement introduces a sumcient time constant'in the automatic gain control'voltage so as to maintain the gain of the amplifier over word and syllable pauses. Since the transformer winding 3|! also supplies filament current to the vacuum tubes 203 and 204, the bias appearing across the resistor 332 is stabilized to a considerable degree so that in spite of the fact that the plate current of the vacuum tube 20] reduces considerably as the automatic gain control bias is applied, the vacuum tube operates with a proper amount of-negative bias potential.

- is obtained from across the voltage divider 269 and by means of a coupling capacitor 297 is applied to the control grid of the control vacuum tube 208, thereby to cause anincrease in the anode to cathode current of the vacuum tube 208. Thecathode circuit of the .control tube 209 may be traced from the midpoint of the filament transformer winding 3l9 through the resistor 339 to the negative side of the plate potential supplied by the rectifier tube 3i I. A change in the anode current of the control-tube 200 therefore rapidly changes the voltage drop appearing across the resistor 339. The resistor 339 is effectively connected in circuit with the grid to cathode circuit of the vacuum tube 206. A circuit from the grid of the vacuum tube 200 may be traced throu h the lower portion of the voltage divider 203, resistors 218 and 2'80, resistor 215 which is shunted by the condenser 220, resistors 336, 339, 393 and 392, and back to the mid-tap of the filament transformer winding 32l which energizes the filament of the vacuum tube 206. The negative bias applied tothe vacuum tube 206 therefore is the drop appearing across resistors 342 and 343 when no current is flowing in the vacuum tube 200. With the increase in the anode to cathode currentof the vacuum tube 208,'however, the negative bias applied by the resistors 342 and 333 is opposed by the bias appearing across the resistor 339. The potential supplied by the resistor 339 is obtained from a resistance capacitance arrangement which has a relatively short time constant so that a rapid enabling action of the vacuum tube 206 is obtained. The midpoint of the transformer winding 3 l 9 is connected through the resistors 339, 343 and 302 to the midpoint of transformer winding 32! which supplies filament current to the. input vacuum tube 206. The change in bias brought about by the-change in the anode current of vacuumtube 208 therefore is applied to the grid to cathode circuit of the input vac --uum tube 206. The voltage appearing across the resistor 339 varies widely from standby condition to enabled'condition, although the voltages appearing across the resistors 342 and 343 do not vary greatly. Since the voltage appearin across the resistor 333 varies considerably in accordance with th change. in anode current of the control tube 200, the negative bias applied to the input vacuum tube 206 is rapidly reduced, thereby quickly increasing the gain of the incoming channel amplifier thereby to enable the amplifier. It will be readily appreciated that the action of the vacuum tube 208 has an action with respect to the input vacuum tube 206 very similar to the action of the control tube 205 with respect to the vacuum tube 202 of the outgoing channel ampli fier. Having the initial increase in gain of the vacuum tube 206, the increased current flowing in the vacuum tube 209 brings about actuation of the relay 283 which closes its contacts 284 and 285, thereby to energize the slow release relay 296. The capacitor 29| which is arranged to be charged through a current limiting resistor 28l is connected across therelay 286, thereby to improve its slow release characteristics. In order to change the bias appearing on the vacuum tube 206 the contacts 294 and 295 are closed, thereby rendering ineffective the variable bias supplied by the action of the vacuum tube 208 and also changing the degree of bias provided by the resistors 342 and 343 so that the'gain of the incoming channel amplifier is stabilized at a new gain level. At the same time contacts 293 and 294 are closed, which place the resistor 292 in shunt with the resistor 343. This gives a combination of voltage drops across the two resistors which is somewhat less than the voltage appearingacross the resistor 325. Thus these voltage drops are opposite and unequal and are in series with the automatic gain control voltage path to ground so as I to produce an automatic gain delay voltage. The

automatic gain voltage which is obtained from the voltage divider 208 by the coupling capacitor 2', the current limiting resistor 21'2 and the dry disk rectifier 213 appears across the capacitor 210. During the initial period of incoming speech and after the closure of the contacts 294'and 296 of the relay 286 the automatic gain g v asoa ve control bias from the capacitor TM is super'im- I posed upon the normal negative bias applied to the vacuum tube 205 by means of the drop apearing across the resistor 342. Operation of the slow release relay 286 also changes the contact 26! from ground to contact 262, thereby connect. ing the grid of the vacuum tube 263 through controlled by said control-tube for subsequently rendering ineffective said second means and a portion of said first means to maintain the gain of said amplifier above apredetermined level.

the resistor 256 to a bias voltage appearing across the voltage divider 255. This negative bias volt-' age is sufi'icient to reduce the gain of-the vacuum tube 2% so as to thereby disable the outgoing channel amplifier.

While theoutgoing channe1 amplifier has been shown utilizing four stages of amplification, it is to, be understood that'either a greater or lesser number of stages may be utilized, and in the present instances four stages of amplification were deemed necessary to supplya slight excess of gain whereas three stages might be insuflicient." .In order to balanceor offset "this excess'gain, a

T-pad comprising the'resistors 32 i to MB is inserted between the conductors 28d and 220 and the winding of the transformer 302 of the hybrid system dill.

If it is desired to interrupt the operation oi the executive loudspeaking communication system to obtain the privacy of the handset, the

closure of the contacts I06 and Hill causes the conductor 'l4ii to be efiectively connected to ground, wherebypower flowing over the conductors 250 and Hill will energize the relay 4 I 3. This disconnects the hybrid system 40! and short-circuits the input to the incoming signal channel. If subsequently communication is to be resumed by the loudspeaking apparatus, the replacement of the receiver to actuate the receiver switch I03 will break the contacts I06 and I01, thereby to deenergize relay M3 and again to place into operation the signal channelamplifiers.

Mention was previously made of the desirability, of providing a system which used a single type: of

' tube and, hence, in the accompanying drawings It, of'

all the tubes are shown as being triodes. course, will be understood that while triodes have been shown, the principles herein disclosed may 2. In a transmission system, a channel amplifier having a plurality of stages of amplification, a control-tube arranged to be responsive to signal currents passing through the output stage of said amplifier, self-biasing means connected to at least one stage of said amplifier for supplying thereto a negative bias sufficient to maintain the gain thereof at a relatively low level, biasing means having a relatively short time constant connected in circuit with said control tube :and

said first means for deriving a control voltage in opposition to a portion of said negative self-bias thereby to increase the gain of said amplifier, and means controlled by said controltube for subse-= quently rendering ineffective said second means and a portion of said first means to maintain the gain of said amplifier above a predetermined level.

3. In a transmission system, a channel amplifier provided with means for supplying thereto a negative bias sufiicient to maintain the gain thereof at a relatively low level, a control tube arranged to be responsive to signal currents passing through said amplifier, means connected in circuit with said control tube and said first means for deriving a control voltage in opposition to said negative bias, said means having a relatively short time constant thereby'to rapidly increase the gain of said amplifier, and means controlled by said control tube responsive to an increase in the gain of said amplifier to a certain level for short circuiting said second means and a portion be applied to amplifiers having tubes whichhave a greater number of elements therein.

In order that the present specification be sufficiently complete and understandable without a reference to other applications, certain circuit arrangements and apparatus have been shown and described to facilitate such understanding. Certain broad aspects of the circuit arrangements and apparatus, herein disclosed but not claimed,

are shown, described and claimed in my copendnow Patent No. 2,369,460, granted February 13,

While one' embodiment of the invention has been disclosed, it will bel'understood that various modifications may be made therein which are within the true spirit and scope of the invention.

What is claimed is; 1. In a transmission system,.a channel amplifier having a plurality of stages of amplification,

ing application for improvements in Transmission systems, Serial No. 483,596, filed April 19, 1943,

of said first means to maintain the gain of said amplifier above a predetermined level.

4. Ina transmission system, a channel amplifier having a plurality of stages of amplification, a control tube arranged to be responsive to signal currents passing therethr'ough, means connectcd to said amplifier for supplying theretoa negative bias s'uflicient to maintain the gain thereof at a relatively low level, means having a relatively short time constant connected in circuit with said control tube and said first means for deriving a voltage in opposition to a portion of said negative bias thereby to increase the gain of said amplifier, means controlled by said control tube for subsequently rendering ineffective said second means and a portion of said first means, and an automatic gain control circuit en ergized from the output stage of said amplifier and having a time constant sufilcient to hold over word and syllable pauses for supplying an automatic gain control voltage'to said amplifier.

a control tube arranged to be responsive to signal currents passing through the output stage of said amplifier, means connected to said amplifier forsupplying theretoa negative bias sufiicient to maintain the gain thereof at a relatively low level, means connected in circuit with saicl control tube and said first means for deriving a control voltage in opposition to said-negative bias thereby to increase the gain of said amplifier, and means 5. In a transmission system, a channel amplifier having a plurality of stages ofamplification, a control tube arranged to be responsive to signal currents passing therethrough, means connected to said amplifier for supplying thereto a negative bias sufiicient to maintain the gain means, and an automatic gain control energized from the output stage of said amplifier and having a time constant sufilcient to hold over word and syllable pauses connected in circuit with said first means thereby to .superpose. an automatic gain control bias on said negative bias.

e. In a transmission system,a channel amplifier including at least an input tube and an output tube, means connected to said amplifier for supplying thereto a negative, self-bias vsufilcient to maintain the gain thereof at a relatively low.

means being connected in circuit with said first means thereby to superpose an automatic gain control bias on said negative bias, and means con-. trolled by the said control tube responsive to an increase in the gainof said amplifier to a certain level for rendering ineffective said second means and a portion of said first means to maintain the gain of said amplifier above a predetermined level apdior delaying the application of an automatic gain control bias until a predetermined signal efi'ectively controlling the gain oi the first stage oi amplification, a' control tube and circuit for saidarnpiifier arranged to derive a control voltage in response to signal currents passing through said amplifier, means for applying said control voltage in opposition to said biasing means to increase the gain of the amplifier, and means controlled by said control tube responsive to an increase in the gain oi said amplifier to a certain level for further reducing the bias of said amplifier to maintain the operation thereof at an increased gain level.

d in a transmission system, the combination of a, multi-stage amplifier with biasing means for maintaining the gain thereof at a relatively low. level. automatic gain control meanshaving an appreciable time constant to hold over word and syllable pauses for effectively controlling the gainof the first stage of amplification, acontrol tubeand circuit for said amplifier energized from the output stage and arranged to derive a control dering ineffective said second control voltage and a portion of the bias of said amplifier to maintain operation thereoi at an increased gain level.

9. In a transmission system, the combination of apair of channel amplifiers for transmittingsignal currents in different directions, negativebiasing means for each channel amplifier to maintain the gain thereof at a predetermined low level,

. a control tube and circuit for each channel amplifier arranged to derive control voltages in response to signal currents passing therethrough.

the gain of said amplifier, means responsive to currents flowing through said control tube for subsequently reducing theamount of negative .bias supplied by said first means, and means controlled by said control tube and circuit for increasing the negative bias of the other channel amplifier thereby to reduce the gain thereof.

10. Ina transmission system, the combination 01' a pair of multi-stage channel amplifiers for transmitting signal currents in, different directions, negative'biasing means for each channel amplifier to maintain the gain thereof at a predetermined low level, an automatic gain control for each amplifier for effectively controlling the ains! the'first stage of amplification, a control tube and circuit for each channel amplifier arranged to derive voltages in response to signal currents passing therethrough, means for supplying said control voltages in opposition to said negative bias thereby to reduce the effectiveness thereof, means responsive to current flowing through said control tube for subsequently reducing the amount of negative bias supplied by said first means, and means controlled by said control tube and circuit for. increasing the negative bias 7 of the other channel amplifier thereby to reduce the gain thereof.

lL'In a transmission system. the combination of a pair of multi=stage amplifiers for transmitting signal currents in difierent directions, each amplifier being provided with biasing means for maintaining the gain thereof at a relatively low level, automatic gain'control means for each amplifier for eiiectively controlling the gain thereof, said automatic gain control means having an appreciable time constant to hold over word and syllable pauses, a control tube and circuit for each I amplifier arranged to derive control voltages in voltage in response to signal currents passing means for applying said control voltages in opposition to said negative bias thereby to increase.

response to signal currents passing therethrough,

means for applyingsaid control voltages in opposition to said biasing means to increase the gain of the amplifier, means controlled by said control tube for subsequently removing said control voltage and reducing the bias of said amplifier, a

source of bias potential, and means controlled by said control tube for connecting said source of bias potential to theother amplifier to reduce the gain thereof. 1

12. In a transmission system, the combination of multi-stage channel amplifiers for transmitting signal currents in difierent directions, each amplifier being provided with self-biasing means for maintaining the gain thereof below a predetermined level, automatic gain control means for each amplifier for effectively controlling the gain of the first stage of amplification. said auto= matic gain control means having atime constant sumcient to hold 'over'word and syllable pauses, a control tube and circuit for each amplifier ar= ranged to derive-control voltages in response to signal currents passing through said amplifier, means for applying said control voltages in oppo sition to said self-biasing means to increase the ain of the amplifier, means controlled by said control tube for subsequently reducing the self= bias of said amplifier, a source or bias potential, and means controlled by said control tube for connecting said source of bias potential to the other amplifier to reduce the gain thereof.

13. In a transmission system, the combination of incoming and outgoing multi-stage amplifiers for transmitting signal currents, each amplifier being provided with self-biasing means for maintaining the gain thereof at a relatively low level. a control tube and circuit for each amplifier argain'of the amplifier, each of said amplifiers being provided with automatic gain control means energized from the output stage and acting upon the input stage of the amplifier, means responsive to the operation of said control tube for subsequently removing the cheat of the control voltage generated thereby and for reducing the selfbias of said amplifier, said means including a relay arranged to short-=circuit a portion of said self-biasing means and said means for deriving control voltages from said control tube, a source of bias potential, and means controlled by said control tube for connecting said source of bias potential to the other amplifier to reduce the gain thereof.

14. In a transmission system, the combination of multi-stage channel amplifiers for transmitting signal currents in different directions, each amplifier being provided with self-biasing means for maintaining the gain thereof at a subnormal level, automatic gain control means'for each amplifier associated with the output stage thereof to derive voltages for efiectively controlling the gain, of the first stage of said amplifier, said gain control means having a'time constant sufand means for applying said derived control voltages to oppose said self-biasing means to increase the gain of the amplifier to a predetermined normal level, means controlled by said control tube comprising an incoming channel amplifier and an outgoing channel amplifier, each amplifier be ing provided with a control tube and circuit arrangedv to derive control voltages in response to signal currents passing therethrough thereby to increase the gain of the amplifier, a source of negative bias potential, means controlled by each control tube for connecting said source of bias potential to the other amplifier to reduce the gain thereof, said outgoing channel amplifier having a plurality of stages of amplification including a first stage provided with self-biasing means,

means energized from the output stage of said amplifier for supplying in series with said selfblasing means or said first stage an automatic gain control voltage having a time constant sufficient to hold over word and syllable pauses, seli2- biasing means associated with the second stage of said amplifier for applying thereto a bias surficient to maintain said amplifier at a relatively low signal level, means connected in circuit with for subsequently removing said control voltage,

' and reducing the self-bias of said amplifier, and

means controlled by said control tube for reducing the gain of the other channel amplifier. 15. In a transmission system, a combination of incoming and outgoing channel amplifiers, each amplifier being provided with a control tube arranged to be responsive to signal currents passing therethrough thereby toincrease the gain of its amplifier and to decrease the gain of the "other amplifier, said incoming channel amplifier said control tube and with said latter seliwbiasing means for deriving a voltage in opposition to said self-biasing means thereby to rapidly increase the gain of said amplifier, a. relay actuated by said control tube, said relay being arranged to subsequently short-circuit a portion of said self-biasing means and said means for generating a control voltage, said relay being arranged to connect said source of negative bias potential to the input stage of said incoming channel amplifier thereby to disable said amplifier, and means associated with the third stage of said outgoing channel amplifier for disabling said amplifier in response'to operation of the control tube associated with said incoming channel amplifier.

17. In a transmission system, the combination comprising an outgoing channel amplifier having a. plurality of stages of amplification, self-biasing said self-biasing voltage, said control circuit havr ing a relatively short time constant whereby the trol tube a voltage in opposition to said self-biasing means thereby to rapidly increase the gain of said amplifier, said control tube'being provided with a relay arranged to short-circuit said control voltage means and a portion of said selfbiasing means thereby to maintain the level of said amplifier at a predetermined amount, means associated with said relay for connecting in series with said automatic gain control voltage sufficient negative bias to delay the operation thereof until a predetermined signal level has been exceeded, a source of negative bias, and means controlled by said relay for connecting said source of negative bias to the outgoing channel amplifier thereby tov reduce the gain thereof sufficiently to render said amplifier inefiective.

16. In a transmission system, the combination gain of said amplifier will be increased at a relatively rapid rate. a relay energized by said control tube, said relay being arranged to short-circuit a portion of said self-biasing means and said means 3 for deriving a, control voltage in opposition to said self-biasing means thereby to stabilize the gain of said amplifier at a predetermined level, an incoming channel amplifier having an input stage and an output stage, self-biasing means associated with the input stage for deriving a. nega-] passing through said amplifier, said control circult having means for deriving control voltages in opposition'to said self-biasing means and having a relatively short itme constant thereby to rapidly increase the gain of 'said amplifier, a relay controlled by said control tube, said relay being arranged to short-circuit said. means for; deriving control voltages and a portionoi said sell-biasing means, said relay also being arranged to supply in series with the said automatic gain control voltage a new negative bias potential sufncient to delay the action of said automatic gain control circuit until the signal level of the amplifier exceeds a certain predetermined value, 30 

